With the aim of improving adhesion, it is known practice to incorporate a formaldehyde acceptor into rubber, such as a meta-substituted phenol (for example resorcinol or an m-aminophenol) and a formaldehyde donor capable of producing formaldehyde by heating in the rubber.
The abovementioned m-substituted phenols are highly volatile. Thus, when they are used to improve the adhesion of rubbers, they release harmful fumes at the vulcanization temperature (generally above 110° C.).
In addition, these formaldehyde acceptors are hygroscopic, which entails the formation of efflorescences in the rubber compounds during storage. Such efflorescences give rise to an unequal adhesion between the rubber and the reinforcing material, which compromises the quality of the vulcanized articles.
To solve these problems, it has been proposed to use a “resorcinol resin” as formaldehyde acceptor, obtained by coupling resorcinol with formaldehyde (Industrial Engineering Chemistry, pp. 381–386, (1946)). Although the resin makes it possible to obtain both substantial adhesion and an improvement in the physical properties of the rubber (hardness and modulus), it has a deliquescent state such that it adheres to the wrapping materials or forms a block. It goes without saying that this poses numerous problems during storage or handling. In addition, the resin contains a large amount of free resorcinol (≧15%), which does not make it possible to overcome the drawbacks associated with the presence of fumes and caking.
EP-A 419 741 and DE-A-4 001 606 have proposed the use of modified novolacs resulting from co-coupling of a phenol (for example resorcinol), an unsaturated hydrocarbon (for example styrene) and an aldehyde (for example formaldehyde).
Ternary products of co-coupling of a phenol such as resorcinol, an alkylphenol and an aldehyde have also been proposed. For example, in FR 2 193 046, FR 2 223 391, FR 2 392 049 and EP-A-602 861, the said products are obtained by coupling a resol (alkylphenol/formal) with resorcinol in the presence of an acid catalyst.
U.S. Pat. No. 5,030,692 describes the synthesis of a ternary compound by coupling resorcinol with a resol obtained by reacting an alkylphenol and methylformcel or furfural in acid medium.
In the ternary products which have just been mentioned, while the free resorcinol content is undoubtedly low (<5%), it is, however, accompanied by a loss of reactivity with respect to the formaldehyde donor.
JP 62-004 720 discloses novolac resins obtained by coupling an excess of resorcinol and dicyclopentadiene (resorcinol/dicyclopentadiene molar ratio=2.9), the excess resorcinol being removed at the end of the reaction, for example, by distlllation under reduced pressure or fractional precipitation. These resins have improved heat resistance and flexibility properties that are desired in the electronics field.
EP-A-798 324 discloses novolac resins obtained by coupling an aromatic compound with at least two hydroxyl groups such as resorcinol and an unconjugated diene such as dicyclopentadiene in an (aromatic compound)/(diene) molar ratio of between 0.7 and 1.5, in which the free resorcinol content is in the region of 5% and in which the number-average and weight-average molar masses are rather low (usually less than 1000).
U.S. Pat. No. 4,889,891 discloses compositions based on vulcanizable rubber in which are dispersed a novolac resin which is obtained by modification of a substituted phenol or resorcinol with a mono- or polyunsaturated hydrocarbon followed by coupling with a monoaldehyde.
U.S. Pat. No. 3,969,321 discloses a process for the preparation of synthetic resins consisting of the reaction of a dienic compound directly with a phenolic resin, particularly of novolac type, in the presence of a strong acid catalyst. Novolac resins are obtained by reacting an aldehyde and a phenol having a sole hydroxyl group.
U.S. Pat. No. 3,538,052 discloses phenol-aldehyde novolac resins reacting with a specific mixture consisting of indene substituted or not, dicyclopentadiene, codimer of cyclopentadiene et at least of a conjugated non-cyclic diene and a compound having at least a phenyl group substituted or not by one or more vinyl(idene) groups; the authors point out that, in comparison with classical novolac resins, these resins have both improved water-absorption and improved mechanical and electrical properties. They can be used as adhesives or binders for granular materials and can advantageously replace known thermosetting pulverulent resins.